[填空题]
Tom uses a compass to draw a circle. The arm b .d+6ik7b0xktw vc 9dof the j.(ela 2k zog-compass with the needle is 7 cm long, and the arm with the pencil o gl.-ze2 (ajkis 8 cm long. The angle between the arms is θ as shown in the diagram below.
1.Find an expression for the radius of the circle to be drawn in terms of θ.
r=$\sqrt{a-b cosθ}$, a= , b= .
2.Hence, find the rate of change in r with respect to θ when θ=30$^∘$ .
the rate of change is cm per radian
